fix: trim trailing spaces in invoice attachment filenames

[miachillgood]

Summary

• trim trailing spaces from fileName before building the invoice attachment path
• apply the same normalization to both create and update invoice attachment-by-filename calls
• add regression coverage that verifies the generated request URL no longer includes trailing spaces

Why

Issue #763 reports that invoice attachment uploads fail when the filename has trailing spaces. Today the SDK forwards the filename verbatim into the path segment, so "invoice copy.pdf " becomes a different attachment path than "invoice copy.pdf". Trimming the trailing whitespace on the client side matches the practical expectation for filenames and avoids the server-side 500 path.

Validation

• npm test -- --runInBand
• npm run build

Closes #763